Fecundability is the monthly probability of conception. It determines the waiting time to conception,thus affecting the length of birth intervals. Both physiological and behavioral factors interact to determine fecundability. A model of these interactions has been developed and validated with data from U.S. women. This proposal extends the model by incorporating new theoretical and empirical developments; it also provides methods for testing model predictions against empirical data on the interval from marriage to first birth. To examine both the physiological and behavioral components of fecundability, several sources of data will be used. Two of these include a large number of menstrual cycles for which daily coital diaries and daily measures of urinary luteinizing hormone will be available. These data will be used to estimate the distribution of intercourse by cycle day, as well as by partners' ages, marital duration, and time since last intercourse. A third data set will be used to investigate heterogeneity in ovarian cycles and its effects on fecundability. The final collection of data comes from a series of surveys in Taiwan, which will provide observed distributions of first birth intervals to be analyzed using life-table techniques. The proposed research addresses several issues concerning contemporary reproductive patterns. The combination of delayed onset of childbearing and truncation of childbearing through surgical sterilization has led to concentration of reproduction in a narrow segment of most women's life spans. As a result, there are rising public concerns about the expected waiting time to next conception, the likelihood of pregnancy loss, and possible problems with pathological infertility. The proposed research provides a theoretical and empirical basis for evaluating these issues. More generally, this research is part of an effort to integrate biological processes into demographic models. The collaboration described in this proposal, with its unusual combination of biological and demographic skills, fosters an increasing dialogue between biologists and social scientists concerned with human fertility.